Pneumatic control of process systems is widely employed wherever the system installation requires rugged components, lowered cost, relative ease of installation and troubleshooting and a high degree of controllability. Examples of such processes which readily lend themselves to pneumatic control include the control of chiller and boiler temperature, steam or air line pressure control, flow control in fluid-transporting pipe systems, tank liquid level control, pH control in chemical processes, and heating, ventilating and air conditioning controls. Pneumatic control is frequently employed in petrochemical process systems where flammable fluids are often present and may be ignited by electrical control devices. For purposes of illustration, and not by way of limitation, the invention is shown and described in connection with a heating, ventilating and air conditioning system.
Heating, ventilating and air conditioning (HVAC) systems are frequently used in buildings to control the temperature of a conditioned space within the building and for energy management purposes. A type of HVAC system includes an air handling unit having a plurality of actuator-manipulated shutters and dampers for controlling the flow of outdoor air into the building, for controlling the flow of air exhausted from the building and for directing air which is heated or cooled and recirculated. Other mechanisms associated with air handling units typically include actuator-manipulated valves for controlling the flow of chilled or heated water through heat exchanger coils disposed in the ductwork for controlling the temperature of air flowing therethrough.
One type of actuator used with such air handling units is of the spring biased, pneumatic cylinder type coupled by a position control device to a source of pneumatic pressure such as a pneumatic bus network formed of small diameter flexible polyeythlene tubing and installed throughout the building. The position control device, typically a pneumatic logic panel, controls the flow of pressurized fluid from the pneumatic bus to the actuator and the flow of exhaust fluid from the actuator to the surrounding ambient for maintaining specified parameters, space temperature being exemplary. Control is by the solution of known algorithms and the generation of analog output signals directed to the actuators.
Optimally, it is desirable in any control loop to accomplish actuator movement in the shortest practical time in order to avoid the unnecessary introduction of error into the system. However, for reasons related to overall installation cost, design features are often incorporated which regulate the stroke time of an actuator. In order to control the time required to stroke an actuator over a predetermined distance and against the opposing force of its biasing spring, an inlet restrictor having an orifice of reduced diameter therethrough may be disposed in the input pneumatic lines coupled to the actuator. A second restrictor may be employed in the exhaust line between the actuator and the surrounding ambient for controlling the actuator stroking characteristics in the opposite direction.
The advent of computerized direct digital control apparatus and the desire of building owners to incorporate such computerized apparatus into new or existing HVAC systems employing low cost, rugged pneumatic actuators requires that a digital-to-pneumatic interface system be employed for receiving digital signals from the control apparatus and translating them to pneumatic signals for actuator positioning. Computerized, direct digital controllers may be constructed and arranged to repetitively solve any one or more of several known control algorithms for generating command signals to the interface system. Examples of such algorithms include the standard proportional integral (PI) and proportional integral derivative (PID) algorithms, the latter usually being presented in its position equation. This equation calculates the position of an actuator at the nth sampling time as a function of the measured error. A disadvantage of using this equation to control pneumatic actuators is that the resulting command signals from the digital controller must be converted to an absolute pneumatic pressure having a known relationship to an absolute actuator position and this signal conversion requires a relatively expensive type of transducer.
For reasons related to lower cost and optimum control, it is preferable to arrange the controller for the solution of the PID algorithm in its velocity form, sometimes called the incremental form. The solution thereof results in controller digital command signals which direct the interface system to effect a computed change in actuator pressure rather than provide a new absolute pressure. This change in pressure results in a change in actuator position, the magnitude of which is a specified percentage of its total available stroke distance. Use of the incremental form of the equation provides bumpless transfer, eliminates a phenomenon known as integral windup and requires no feedback device for continuously indicating actuator position. However, the use of the incremental form of the PID algorithm requires that the interface system provide precise actuator positioning in a manner unaffected by any change in the volume of fluid contained within that portion of the pneumatic system comprising the actuators and the bus connected directly thereto. An example of an interface system useful for controlling the position of a single actuator or for the simultaneous control of the position of several actuators of the same size, spring range and loading is shown in U.S. Pat. No. 4,261,509. This system includes a pair of two position, electrically actuated solenoid valves for receiving digital signals and controlling the flow of fluid into and out of the actuator. Pneumatic resistors, sometimes termed restrictors, having orifices therethrough typically of a few thousandths of an inch in diameter are disposed in the pneumatic lines for controlling actuator stroke distance per unit time, i.e., for controlling the slopes of the actuator pressure-time graphs representative of actuator stroke characteristics in both directions of travel.
For air handling units having a plurality of actuator-positioned dampers, shutters and valves, it is important for energy conservation reasons that those devices be properly sequenced to utilize, insofar as possible, the cooling capability of the outdoor air. For example, as the temperature in a conditioned space rises, the actuator controlling the position of a hot water valve would be positioned to move the valve to a more flow-restrictive position. If space temperature continues to rise, the hot water valve would be modulated to a closed position. Simultaneously, outdoor air, exhaust air and return air dampers would be positioned by their associated actuators to permit, respectively, the introduction of additional outdoor air into the conditioned space, the exhaustion of a similar quantity of air to the ambient and the restriction of the flow of return air moving between the outdoor air duct and the exhaust air duct. If the temperature of the outdoor air is insufficiently low to satisfy space cooling requirements and after the outdoor air damper is fully opened, the chilled water valve is modulated to a less restrictive position to result in additional cooling to the space. The importance of closely controlled sequencing of the various actuators is apparent.
One solution to the problem of actuator sequencing is to design or select actuators having spring ranges which result in sequential actuator operation, from minimum to maximum position, over a predetermined increment of pressure within the range of the available control pressure. In the aforementioned example and assuming an available control pressure range of 0-20 psig, the actuator controlling the heating coil valve may have a spring selected to permit full actuator stroking over the 3-8 psig increment, the outdoor, return and exhaust damper actuator springs selected to permit stroking over the 8-13 psig increment and the chilled water valve actuator spring selected to permit stroking over the 13-18 psig increment. If actuators having only fixed or unknown spring ranges are available, a far more typical circumstance, proper sequencing may be achieved by incorporating pilot positioners upon all valve and damper actuators. Such positioners are mechanically coupled to the associated actuator, have closed pressure chambers of known volumes and have adjustments for selecting the pressure at which spring-compressing actuator movement commences and the pressure range or span over which total actuator travel may be effected. The pilot positioner is thereby capable of positioning its associated actuator from a selectable pressure starting point and over a selectable pressure span, each of which is independent of the size, spring range of and loads on the actuator.
While interface systems of the aforementioned type have heretofore been satisfactory for the positioning of actuators they are nevertheless characterized by certain disadvantages. For example, when restrictors are used to control the stroking characteristics of a single actuator or of a group of actuators having the same size, spring range and loading, the restrictor orifice sizes must be selected by experimentation at the installation site. This is so since actuator stroke times are dependent upon actuator size, spring range, loading and the volume of fluid contained within the actuator and the pneumatic interconnections. These parameters are frequently difficult or impossible to determine prior to actual installation.
If the HVAC system requires actuator sequencing and incorporates actuators having different volumetric sizes, spring ranges and/or loadings, the system will exhibit highly nonlinear gain characteristics and the control problem is further complicated. Using the interface system of the aforementioned patent as illustrative, and assuming a plurality of parallel connected dissimilarly-configured actuators to be controlled, the percent change of position will be different for each actuator for a given time during which a solenoid is energized for introducing fluid to or expelling fluid from the actuators. This is so since a change in the contained volume of fluid of one actuator will affect the stroke distance per unit time of other actuators in accordance with the equation of state of an ideal gas. If restrictors are selected to control the stroke time of, for example, a small, lightly loaded actuator, the system response will be unacceptably sluggish for positioning larger or more heavily loaded actuators. Conversely, if restrictors are selected for the proper control of actuators of the latter type, system instabilities may result. Even with the addition of pilot positioners to some or all of the actuators, restrictor selection must be by field experimentation or by measurement and computation of the volume of compressed fluid contained within the pneumatic interconnections and the pilot positioner pressure chambers.
A further refinement of the system described above includes the addition of a constant volume reservoir coupled in parallel with the pneumatic bus to which the actuators are connected. If pilot positioners are used on all actuators and if the capacity of the reservoir is selected to confine a volume of fluid which is, for example, at least ten times that of the volume of fluid contained within the constant volume chambers of the pilot positioners and within the interconnecting pneumatic tubing, the restrictors may be preselected and the gain of the resulting system will vary less than 10%. However, 100 feet of 1/4 inch OD polyethylene tubing as commonly employed in pneumatic bus networks has a contained volume of about 27 cubic inches. If this contained volume is to affect actuator stroke times by less than 10%, the confined volume of the reservoir must be greater than 270 cubic inches in order to permit preselection of the restrictor orifice sizes. A reservoir of such size is unacceptably large and, further, it may be impractical or impossible to determine the length and size of the pneumatic tubing prior to actual installation. Additionally and even though restrictors were preselected based upon the known volume of fluid confined in the pneumatic tubing and the input pressure chambers of the pilot positioners, it will be necessary to select restrictors having orifices of different sizes if the configuration of the system and therefore the total confined volume is changed by the later addition or deletion of actuators and/or tubing. Further, the aforedescribed refined system must be used with pilot positioners upon all actuators even though those actuators, because of their spring ranges, may be sequenced without them. This is so since a movement of one or more actuators will otherwise result in a change in the total volume of fluid contained within the system and a consequent change in actuator stroking characteristics.
A further disadvantage of systems of the aforementioned type is that they are susceptible to significant leaks of pneumatic fluid. For example, each pneumatic connection of 1/4 inch tubing typically has a leak rate of approximately 0.1 standard cubic inches per minute (SCIM) at 20 psig while a typical pilot positioner has a leak rate of 0.3 SCIM. In a system including a constant volume reservoir where the system contains a relatively small volume of fluid and/or a large number of connection points and pilot positioners, changes in the control pressure due to leaks within the system and over the time interval between parameter sample times, e.g., conditioned-space temperature sampling times, would be unacceptably large. In accordance with the incremental form of both the PI algorithm and the PID algorithm, a change in control pressure due to leaks would result in a constant offset from the parameter set point, e.g., the desired space temperature, pre-established within the system controller data base and poor control performance will result. While the magnitude of change of the control pressure between sample times may be reduced by the selection of a reservoir having a volume sufficiently large to make the change in system pressure resulting from fluid leaks to be small, this approach similarly requires the selection of a reservoir having an unacceptably large volume and therefore physical size.
Accordingly, a interface system which permits preselection of restrictor orifice sizes irrespective of the configuration of the related pneumatic bus and actuators, which may be used to control actuators having a wide variety of contained fluid volumes and which may be used to position actuators irrespective of whether pilot positioners are used therewith would be a significant advance over the prior art.